The thinhorn sheep (Ovis dalli) is a medium-sized North American sheep with a stocky body and light brown, deeply ridged horns. The male and female thinhorn sheep are easily distinguished from one another. The male, known as a ram, has large, long, curly horns with sharp tips, which can grow up to a metre in length. In contrast, the female, known as a ewe, has small, slender horns, which are gently curved and grow up to around 25 centimetres. The difference in the size of the horns accounts for a large difference in body mass between the male and female thinhorn sheep, with the male’s horns usually comprising between eight and ten percent of its total weight.
There are two recognised subspecies of thinhorn sheep, Dall’s sheep (Ovis dalli dalli) and Stone’s sheep (Ovis dalli stonei), which are differentiated by their colouration. Dall’s sheep has a creamy white coat and tail, whereas Stone’s sheep is black or grey with a black tail. Intermediates between subspecies are also known to occur. The fur of both subspecies is composed of a fine wool undercoat, covered by an overcoat of stiff, long hairs. The body of the thinhorn sheep is stocky, the legs are slender and the tail is short.
The diet of the thinhorn sheep consists mainly of grasses, shrubs and sedges, as well as lichens, willows and mosses at the beginning of autumn when other vegetation is in short supply. ‘Mineral licks’, rich in essential minerals like calcium, can also be important to the thinhorn sheep in certain seasons.
The thinhorn sheep is a gregarious species, with males forming groups of around 15 individuals, known as ‘bands’, and females gathering in larger groups which are accompanied by juveniles.
The mating season of the thinhorn sheep generally begins between November and December, when ‘rutting’ between males starts. This is where bands of males begin to engage in violent confrontations to compete for females, involving kicking, jumping, mounting and slamming the horns together. As an adaptation to these contests, male thinhorn sheep have air spaces in their skulls, which absorb the impact when the horns clash together. Fights are normally won by larger, older males, establishing a dominance hierarchy.
Male thinhorn sheep curl their lips to help detect the scent of any receptive females in the area. Once a female in oestrus has been detected, the male, having removed all potential mates from the area, follows and kicks the female to test her receptiveness to mating. If this is successful, mating occurs, which is followed by the male guarding the female for the next two or three days, to prevent the female from breeding with any other potential mates.
After a gestation period of around four and a half months, a single lamb is born between mid May and early June. The precocial lambs are able to travel with the female only 24 hours after birth, and within days the ewe and lamb rejoin the group. After three to five months, the lambs are fully weaned.
The male thinhorn sheep may occupy up to six different home ranges throughout the year and the female occupies up to four, although some populations are sedentary. As the seasons change, the thinhorn sheep move to more suitable areas. For example, the females will move to specific areas to lamb, usually in secluded, highly elevated areas. The ability to use upland meadows and scale near-vertical cliffs gives the thinhorn sheep an advantage when finding safe areas to give birth and when escaping from predators.
Between April and September, the horns of the thinhorn sheep grow in length and circumference. This creates a deep ring around the horns each year, which can be used to determine the individual’s age. There is also a seasonal moult from March to July, with males moulting earlier than females and juveniles.
The thinhorn sheep is predated by many species, including wolves (Canis lupus), brown bears (Ursus arctos), wolverines (Gulo gulo), coyotes (Canis latrans)and lynxes (Lynx canadensis). Golden eagles (Aquilachrysaetos) are also known to take lambs in the first few weeks of life.
The average lifespan of the thinhorn sheep is around ten years.